Nitrogen isotope variations along the northeast Pacific margin@en

Glacial-interglacial changes in sedimentary d15N over the last 120 kyr display a remarkably similar pattern in timing and amplitude in core records extending from the denitrification zone in the eastern tropical North Pacific (ETNP), where subsurface denitrification is active, to the Oregon margin, where no denitrification occurs today. Low d15N values (4-6 per mil) generally characterize glacial stages 2 and 4, and higher d15N values (7-10 per mil) are representative of the Holocene, millennial-scale periods within stage 3, and stage 5. The inferred synchroneity of d15N variations along the entire margin implies that the nitrate isotopic signal produced in the oxygen-poor subsurface waters in the ETNP is rapidly advected northward and recorded at sites far beyond the boundaries of the modern denitrification zone. Similar to d15N, primary production indicators (percent Corg, Ba/Al, and percent opal) show glacial-interglacial as well as millennial-scale variations along the NE Pacific margin, with higher primary production during warm periods. However, the relative phasing between d15N and paleoproduction tracers within individual records changes latitudinally. Whereas d15N and primary production vary approximately synchronously in the midlatitudes, production lags d15N in the ETNP by several kiloyears. This lag calls for a new understanding of the processes driving denitrification in the ETNP. We suggest that oxygen input by the Equatorial Undercurrent as well as local organic matter flux controls denitrification rates in the ETNP. Moreover, the differences in relative timing point to a time-transgressive development of upwelling-favorable winds along the NE Pacific margin after the last glaciation, with those in the north developing several kiloyears earlier.过去12万年（kyr）以来沉积物d15N（sedimentary d15N）的冰期-间冰期变化，在从次表层脱氮（subsurface denitrification）作用活跃的东热带北太平洋（eastern tropical North Pacific, ETNP）脱氮区（denitrification zone）延伸至现今无脱氮作用的俄勒冈陆缘的全部岩心记录中，均呈现出极为相似的时间进程与振幅模式。冰期2期与4期通常以较低的d15N值（4‰~6‰）为特征，而较高的d15N值（7‰~10‰）则对应全新世、3期内的千年尺度周期以及5期。沿整个陆缘的d15N变化被推断具有同步性，这表明东热带北太平洋贫氧次表层水中形成的硝酸盐同位素信号可被快速向北平流输送，并在远超现代脱氮区边界的站位被记录下来。与d15N类似，初级生产力指标（有机碳百分比percent Corg、钡铝比Ba/Al与蛋白石百分比percent opal）在东北太平洋陆缘同样表现出冰期-间冰期及千年尺度的变化，且暖期的初级生产力更高。不过，单条记录中d15N与古生产力示踪剂之间的相对相位关系存在纬度差异：在中纬度地区，d15N与初级生产力大致同步变化，但在东热带北太平洋，初级生产力滞后d15N数千年。这一滞后现象要求我们重新认识驱动东热带北太平洋脱氮作用的过程。我们提出，赤道潜流（Equatorial Undercurrent）携带的氧气输入以及局地有机质通量共同控制着东热带北太平洋的脱氮速率。此外，相对时间的差异表明，末次冰期后东北太平洋陆缘的有利上升流风场呈现穿时发育特征，北部区域的风场发育时间较南部早数千年。